This invention relates to a video tape recorder and more particularly to a video tape recorder of the helical scan type which reproduces the video signal on the magnetic tape by means of two rotational video heads which are mechanically displaced by 180.degree. from each other.
Conventionally, in a video tape recorder of the helical scan type, during recording, the speed and phase of the video heads are controlled to record certain portions of video signals at a particular position on the magnetic tape, and the longitudinal speed and position of the magnetic tape are also controlled. A recorded video track is oblique to the longitudinal direction of the tape. Every video track includes information corresponding to more than one field of video signals, so as to prevent drop out of reproduced signal at playback.
During playback, the speed and phase of the video heads are controlled, and the speed and position of the tape are controlled to well track the video tracks on the tape. Consequently, each video head alternately tracks the adjacent video tracks on the tape, and video signals reproduced by each video head include more than one field signal. That is, during a certain angle of head revolution, the same video signal is reproduced by both video heads. Then, the video signal reproduced by each head is chosen by head switching means every 180.degree. of the head rotational angle.
In prior art systems, there are two types of video tape recorders, each having a different method of choosing the video signal reproduced by the two video heads every 180.degree. of the head rotation. One of these is a two sensor type and the other is a single sensor type. In the two sensor type, a permanent magnet segment which is rotated with the video heads, is arranged at the position corresponding to one of the video heads. Two sensors for sensing the magnetic flux variation of the magnet segment are mechanically displaced at predecided positions by about 180.degree.. Output signal waveforms of the sensors are individually shaped by shapers. The phases of the shapers output signals are respectively adjusted by monostable multivibrators to eliminate mechanical errors which occur upon assembling the magnet segment and the sensors. An R-S flip-flop is set by the output signal from one of the monostable multivibrators, and reset by the output signal from the other monostable multivibrator. The video signal reproduced by each video head is chosen in accordance with the output level of the R-S flip-flop.
In the single sensor type, two magnet segments, which are rotated with the video heads, are respectively arranged in positions corresponding to the positions of the video heads. The surface magnetic polarities of the magnet segments are different from each other. The sensor for sensing the polarity of each of the magnet segments, is placed at a predecided position. The output signal from the sensor is supplied to a polarity discriminator which discriminates the polarity sensed by the sensor. The discriminator generates two pulse trains corresponding to the polarities N and S. The two output pulse trains correspond to respective magnet segment positions. The phases of the output signals from the discriminator are respectively adjusted by the monostable multivibrators to eliminate the mechanical errors which occur upon assembling the sensor and magnet segments. An R-S flip-flop is set by the output signal of one of the monostable multivibrators and reset by the output signal of the other monostable multivibrator. The video signal reproduced by each video head is chosen in accordance with the output signal level of the R-S flip-flop in the same manner as in the two sensor type.
A significant disadvantage of the conventional video tape recorders mentioned above, either the single or two sensor type, is a requirement of individually adjusting the phases of the output signals from the shapers or the discriminator by means of two monostable multivibrators to obtain the signal for choosing the video signal reproduced by the video heads.